Ultra-high molecular weight polyethylene fiber (UHMWPE), also known as high-strength and high-modulus polyethylene fiber, refers to the high performance fiber produced from polyethylene having a relative molecular weight of 1×106 or higher through sequential processes of spinning, extracting, drying and ultra-drawing. A fiber reinforced composites produced by using ultra-high molecular weight polyethylene fiber have the advantages of light weight, good impact resistance, high dielectric property and the like, and are widely used in the fields of aeronautic and astronautic industries, the defense in sea areas, weapon facilities and ordinary industries.
In the prior art, the gel spinning technology, a technology firstly invented by DSM Corp., Holland, are generally adopted in the preparation of ultra-high molecular weight polyethylene fiber. In this technology, polyethylene with relative molecular weight of 1×106 or higher is usually used as raw material, the suspension obtained by mixing and swelling the material in suitable solvent is used as spinning solution, then the spinning solution is sheared, homogenized and disentangled by screw extruder and then extruded through spinneret assembly to obtain the as-spun gel filament, after the subsequent extracting, drying and ultra-drawing of the as-spun gel filament, ultra-high molecular weight polyethylene fiber is obtained.
To ensure that the as-spun gel filament possesses sound drawing capacity so as to achieve superior mechanical behavior, it is necessary that there exist a certain number of entanglement points in the gel filament, that is, it is necessary to increase the density of the chain entanglement points in the as-spun gel filament. Because the macromolecular entanglement density of ultra-high molecular weight polyethylene fiber is closely correlated with the concentration of the spinning solution (solid content) and the molecular weight of the raw material, it is possible to increase the chain entanglement density in the gel filament by increasing, at the time of spinning solution preparation, the molecular weight of the raw material or the density of the spinning solution. However, in practice, the only increase of the molecular weight or the solid content in the spinning solution will bring negative effect. For example, when the molecular weight is too high, the polymer particles will be difficult to dissolve thoroughly during the dissolution process and the phenomenon of caking may occur, causing the spinning solution nonhomogeneous and in turn influencing the performance of the filament. Furthermore, the nonhomogeneity of the spinning solution will also influence the increase of the density of the spinning solution.
In the prior art, in order to prepare homogenous spinning solution, some people resort to stepwise swelling of the ultra-high molecular weight polyethylene, that is, realizing pre-swelling and swelling sequentially by stepwise mixing the ultra-high molecular weight polyethylene in multiple swelling kettles and finally putting them into a dissolving kettle with temperature close to the dissolving temperature of the ultra-high molecular weight polyethylene so that the ultra-high molecular weight polyethylene dissolves and the spinning solution forms. However, when the ultra-high molecular weight polyethylene dissolves, the viscosity of the spinning solution tends to increase dramatically, and the weissenberg effect (or known as “rod climbing effect”) tends to occur, thus influencing the smooth proceeding of the subsequent spinning.
In addition to the above stepwise swelling process, in U.S. Pat. No. 4,422,993 and U.S. Pat. No. 4,430,383, Pole. Smith, Pieta Yang Lemstra etc. utilize two types of polyethylene having a molecular weight of 1.5×105 and 4×106 respectively to prepare the spinning solution, in which case, the solid content can only reach 5%. Although the problem of nonhomogeneity is resolved, the problem of low spinning efficiency emerges. In Patent No CN101525778A, Xin Zhirong etc. start from the polyethylene with molecular weight of 2˜5×106 and make use of screw to realize one-step swelling and dissolving, and the solid content can reach 10˜15%. This method shows certain effect in resolving the problem of nonhomogeneity, but there are higher demands to the process and equipment and the operating procedure is complicated.